The amount of fuel gas consumed in furnaces across a process complex depends on the quality of the fuel gas itself. Control devices adjust the flow of the fuel gas, either directly or indirectly, in order to maintain or achieve a desired coil temperature within a furnace. For example, if the calorific value of the fuel gas in the header system decreases, the coil temperature of a furnace is usually maintained by simply burning a larger amount of fuel gas. However, increasing the amount of gas extracted from the header system can disturb the pressure of the system.
Header pressure can be controlled by manipulating the flow of gasses into the header system (e.g., enrichment gasses and off-gasses). The relationship between the flow of gasses into a header system and the quality of the gasses within the header system depends on many factors, including the amount and quality of any gasses flowing into a header system as well as the volume, quality, and pressure of the gasses within the header system itself. These various factors typically vary continuously, rendering the relationship completely non-linear. This non-linear relationship typically applies to every enrichment stream and off-gas stream entering the fuel gas pool.
Existing feedback control systems for stabilizing fuel gas header systems are unable to control both the pressure and quality of the fuel gas leaving the header system. These control systems only attempt to control the pressure of the header system since excessively low pressure can result in unplanned shutdown of a production complex. To date, practitioners in the art have just accepted unstable header systems because it is either difficult to, or they are simply unable to, control both the pressure and quality of the fuel gas in a header system, to control the dynamic and steady state interaction in the fuel gas pool, to control the non-linear relationships, and to control the balance of fuel gas around a header system in order to control the pressure of the header.
A need exists for solutions that overcome the above-mentioned problems and can stabilize both the pressure and quality of a fluid in a utility system.